CDI is a better set up for less headaches. The Bluetec stuff adds complication and the pumps and tanks are common issues. They are not what I would call super reliable. I am 6 years into owning a 2008 R320. Love it, but it does cost probably three times what a VW does to keep. Parts are the biggest challenge, I do my own repairs and it is not easy tracking down parts, used stuff is impossible to find and new stuff is not that well supported for them. I worked in autoparts most of my life at the wholesale level, there is not much out there for them.
I have had no unexpected repairs and been able to avoid a few common ones just by learning about them-oil cooler seals never been an issue on mine cause I keep clean filters and frequent servicing (there are some theories out there about why the oil cooler seals go). Steering rack input shaft seal I had to do, that was $1600. Needed a very large MB dealer before they understood how and what, this leak is well known on the forums. Suspension bushings are going at 165k, not all of them are available without buying complete control arms or assemblies, or you have to settle for unknown Chinese parts. Some exhaust sensors need cleaning every couple years to get the soot out and read properly, reset the codes and ok for another couple years. It is just the expense of most things that few consider when buying one, and hence common to see many thing wrong with them when looking at used ones for sale. Too complex and costly for most.
